There has been considerable effort expended in developing tools for the nondestructive analysis of materials. In the past, many devices using electrical, optical and acoustic detection systems have been developed for evaluating various parameters in a sample.
More recently, a new approach has been developed wherein information is derived by detecting thermal waves which have been generated in a sample and interact with various thermal features in the sample. Such an approach is described in "Thermal Wave Imaging" by Allan Rosencwaig, Science Magazine, Volume 218, p. 223, 1982, incorporated here by reference.
In a thermal wave system, a beam of energy, usually a laser or an electron beam, is focused and scanned across the surface of the sample. The beam is intensity modulated at a frequency in the 10 Hz to 10 MHz range. As the beam scans across the sample, it is absorbed at or near the surface and a periodic heating results at the beam modulation frequency. This periodic heating is the source of thermal waves that propagate from the heated region. The thermal waves are diffusive waves similar to eddy current waves, evanescent waves and other critically damped phenomena that travel only one or two wavelengths before their intensity becomes negligibly small. Nevertheless, within their range, the thermal waves interact with thermal features in a manner that is mathematically equivalent to scattering and reflection processes of conventional propagating waves. Thus, any features on or beneath the surface of the sample that are within the range of these thermal waves and that possess thermal characteristics different from their surroundings will reflect and scatter the thermal waves and thus become visible.
By measuring the thermal waves in a sample, a variety of surface and subsurface features can be evaluated. For example, minor lattice disruptions which are generally not detectable with conventional optical and acoustic probes can be detected with a thermal wave analysis. Other thermal features, such as mechanical defects, cracks, voids and delaminations can be detected. Thermal wave imaging also offers the opportunity for nondestructive depth profiling and determination of thin film layer thicknesses.
A number of approaches have been suggested for detecting these thermal waves. The first approach included the measurement of acoustic wave carriers that are generated by the thermal waves. This approach was described in U.S. Pat. No. 4,255,971, issued Mar. 17, 1981, to Rosencwaig. The above described technique, while accurate, is a "contact" technique, which requires the connection of a piezoelectric transducer to the sample.
More recently, there has been developed a number of noncontact thermal wave measurement techniques. In one technique, a radiation probe is directed within the periodically excited area on the surface of the sample in a manner to be specularly reflected. This probe beam will undergo periodic angular displacements because of the periodic local angular changes in the surface conditions of the sample induced by the presence of the thermal waves. These periodic angular displacements can be detected, using a split or bicell photodetector. This approach is described in U.S. Ser. No. 401,511, filed July 26, 1982, and now U.S. Pat. No. 4,521,118. The preferred embodiment of this approach is described in U.S. Ser. No. 481,275, filed Apr. 1, 1983, and now U.S. Pat. No. 4,522,510. The latter patents are incorporated herein by reference.
Another noncontact technique has recently been developed which includes the detection of changes in the reflectivity of a probe beam. More specifically, the index of refraction of a sample will vary as the sample is periodically heated. Accordingly, if a probe beam is reflected off the surface of the sample, a reflected probe beam will vary in intensity in a manner corresponding to the changes in the index of refraction of the surface of the sample. Since the changing index of refraction is a function of the changes in the surface temperature induced by the thermal waves, by detecting changes in the intensity of the probe beam, the thermal waves can be detected. A device for detecting thermal waves based on the changes in optical reflectivity is described in copending application, U.S. Ser. No. 612,075, filed May 21, 1984, assigned to the same assignee as the subject invention and incorporated herein by reference.
Recently, devices similar to the ones disclosed above have also been used to generate and detect plasma waves in a semiconductor. More specifically, if an intensity modulated energy beam is focused on the surface of a semiconductor, an electron-hole plasma can be created. This plasma can exhibit wave-like characteristics, as described in "Thermal and Plasma Waves in Silicon" by Jon Opsal and Allan Rosencwaig.
As described therein, the plasma density at the surface of the sample will vary based on the sample characteristics. Furthermore, the variations in plasma density will affect the refractive index at the surface of the sample. The changing refractive index can be measured utilizing some of the noncontact techniques which have previously been successful in the measurement of thermal waves. More specifically, a radiation probe can be reflected off the surface of the sample and changes induced in the radiation probe by the plasma induced changes in the refractive index can be monitored to obtain information about surface and subsurface characteristics of the sample.
Plasma density analysis can be used to evaluate ion dopant characteristics and other features which vary across the sample and also as a function of depth beneath the sample surface. An apparatus for detecting plasma density variations is described in copending application, U.S. Ser. No. 707,485, filed March 1, 1985, assigned to the same assignee as the subject invention and incorporated herein by reference.
The measurement techniques described above are extremely sensitive and suitable for microscopic applications. In each case, changes in either the intensity or angle of reflection of a probe beam which has been "specularly reflected" are measured. Because specular reflection is relied upon, the incoming angle of the probe beam and the location of the detector to capture the reflected probe beam must be accurately controlled. Where the sample has considerably varying surface topology, maintaining accurate control of the detector position, so as to always capture the reflected beam, can be difficult. For example, where the sample is nonplanar, the orientation of the probe beam and detector must be continuously varied as the probe beam is scanned over the surface of the sample. In addition, samples having relatively rough surfaces will cause a large percentage of the beam to scatter, thereby reducing the percentage of the probe beam that will exhibit specular reflection.
Accordingly, it would be desirable to develop an alternate technique for detecting thermal and/or plasma waves which did not rely on specular reflection of a probe beam. Preferably, the alternate technique would also be of the noncontact variety to permit evaluation in a manufacturing situation.
Accordingly, it is an object of the subject invention to provide a new and improved apparatus for detecting thermal and/or plasma waves.
It is another object of the subject invention to provide a new and improved apparatus for evaluating surface and subsurface features in a sample.
It is a further object of the subject invention to provide a new and improved apparatus for measuring thermal and/or plasma waves by detecting the optical scattering of a probe beam.
It is still a further object of the subject invention to provide a new and improved apparatus for detecting thermal and/or plasma waves which can be utilized in situations where measurable specular reflection of a probe beam is difficult to obtain.